Loop impedance meters are used to measure the impedance of the phase-earth circuit. This is done to ensure that, under a fault condition, the resultant current will be large enough to trip the circuit breaker or to blow the supply fuse. Many modern installations have an additional safety device known as a residual current device (RCD). RCDs effectively compare the current flowing in the phase and neutral circuits, and, if the current to earth should exceed a preset value, disconnect the supply. The phase-earth loop impedance measurement inherently has a current flow from phase to earth and hence the phase and neutral currents cannot be balanced. The problem here is that conventional loop impedance meters for measuring the phase-earth loop impedance tend to trip the RCD, which puts the supply out of service until the RCD can be reset. This prevents any repeat of the loop impedance measurement until the RCD has been reset, and it is also generally inconvenient.
Most conventional loop impedance meters connect a load resistor of about 10 ohms from phase to earth and measure the voltage between these connections both under load and without load. This measurement is usually made over one or more mains cycle; for 50 Hz mains frequency, the period between cycles is 20 milliseconds, and the measurement is typically made over 20 to 40 milliseconds. The load current that flows is typically about 25 amps. The loop impedance is typically up to about 1 ohm, and clearly a 1 ohm loop impedance would result in a change in voltage from no load to load of about 25 volts, at a nominal mains voltage of 230 volts r.m.s. This very large earth current for this length of time will trip the RCDs in the circuit, which is very inconvenient when checking the wiring and which interferes with equipment already in use.
Some loop impedance meters use short pulses of large current, typically about 25 amps with a pulse length of about 5 microseconds. These short pulses usually do not trip any RCD in the circuit, but the large currents result in large signals to be measured. In one example, the loop impedance meter applies a large step voltage to the phase-earth loop and measures the resultant current. The voltage step is typically about 80 volts lasting for a few microseconds. All these short pulse loop impedance meters suffer from gross inaccuracy due to the inductive component in the wiring of the mains circuit, which can be substantial and unpredictable.
By way of illustration, inductance values from 100 to 600 μH have commonly been observed, and occasionally even larger values have been observed. The time constant of a circuit of inductance L and resistance R is L/R seconds, and in a typical earth-phase measurement circuit the resistance load is 10 ohms. Since the wiring resistance is usually 1 ohm or less, the circuit resistance can be approximated to 10 ohms. Thus if the inductance is 10 μH, the time constant is 1 microsecond and the time taken to settle to within a 1% error for measurement is 5 time constants, or 5 microseconds. This would usually be acceptable, as 5 microseconds would not be sufficiently long to trip the RCD even with a current of 25 amps. However, with larger values of inductance, the time increases proportionately, and a value of inductance of 100 μH would require a settling time of 50 microseconds which could easily cause the RCD to trip.